The present invention is directed to an apparatus for holding semiconductor wafers, the component parts of which are held together by a mechanical joint. More specifically, the present invention is directed to an apparatus for holding semiconductor wafers, the component parts of which are held together by a mechanical joint that can withstand the harsh conditions of devices used in coating semiconductor wafers.
Processing of semiconductor wafers involves harsh conditions such as exposure to corrosive conditions, high temperatures and rapid thermal cycling. Accordingly, wafer support fixtures, also known as furniture or wafer boats, need to withstand such harsh conditions. One method for processing semiconductor wafers involves rapid thermal processing (RTP). Such processes are performed in rapid thermal annealing apparatus (RTA). Semiconductor wafers are treated in an RTA from room temperature to temperatures of about 400xc2x0 C. to about 1400xc2x0 C. in periods of time on the order of a few seconds. The ability of such RTA systems to rapidly heat and cool a wafer from room temperature to such high temperatures in period of up to 10 seconds make them attractive for use in chemical reaction processes such as epitaxial film, amorphous silicon or polycrystalline silicon deposition.
The semiconductor industry has recognized that silicon carbide can withstand the harsh conditions involved in semiconductor processing and is a superior material for construction of wafer fixtures such as boats. Prior to silicon carbide, quartz was used as a material for wafer fixtures. However, quartz was an inadequate material for wafer fixtures because of the harsh process reaction environment as in RTP systems and the thermal incompatibility with materials used in wafer manufacture.
U.S. Pat. No. 4,978,567 to Miller discloses a silicon carbide wafer fixture employed in an RTP system. The fixture of the Miller patent consists of silicon carbide and is fabricated by chemical vapor deposition of the silicon carbide on a graphite substrate followed by destructive oxidation to remove the graphite. The Miller fixture is a single piece of silicon carbide, including a wafer support surface formed integrally with an annular surface surrounding the wafer support, and further including an annular sidewall for holding the wafer support surface at the proper height.
In the Miller fabrication method, the graphite interfaces with deposited silicon carbide is always formed on the backside of the wafer support section, opposite the support face designed for contact with the semiconductor wafer. As a result, there is no convenient technique for providing such a wafer support face with a precisely planar finish. Also, the Miller process does not allow the mold to be used for providing precisely detailed structural features in the support face.
U.S. Pat. No. 5,538,230 to Sibley discloses a single piece silicon carbide wafer boat that may hold multiple wafers for bulk processing. The wafer boat is a generally cylindrical shell section with an average inner radius slightly greater than the radius of wafers that are to be held in the boat. The generally concave inner surface of the boat includes at least two longitudinally uniform convex portions wherein a plurality of orthogonal slots or grooves are located to provide wafer support. Since the carrier is used in a horizontal position, each of the wafers is thereby supported in a vertical position, parallel to each other. The boat walls have a substantially uniform thickness except for the areas where the wafer slots are located.
The boat is made by chemically vapor depositing (CVD) silicon carbide on a graphite mold. The resulting silicon carbide shell is separated from the graphite mold by destructively burning away the mold whereby only the deposited shell remains. The orthogonal slots or grooves are machined into the shell to provide the wafer support points. Other features of the boat, such as length, height and width of the bottom and base widths may be shaped by grinding. Although post-deposition machining of a monolithic CVD-silicon carbide sheet or block may be used to form the desired object, such machining is difficult. Silicon carbide, especially theoretically dense (entirely non-porous) CVD silicon carbide, is very hard and renders machining difficult and costly. Thus, a silicon carbide boat that may be employed for bulk processing of wafers with a minimal amount of machining is highly desirable.
Fabricating semiconductor furniture from a plurality of CVD silicon carbide parts also presents a number of difficulties. Specialized applications to which CVD silicon carbide articles are often employed require that any bonds between the parts withstand extremes, such as temperature extremes. Thus, in fabricating semiconductor furniture from a plurality of CVD silicon carbide parts substantially all organic-based adhesives are entirely unsuitable because they decompose far below the semiconductor processing temperatures.
Several techniques have been proposed to bond silicon carbide parts or components. These include direct bonding (T. J. Moore, xe2x80x9cFeasibility Study of the Welding of SiCxe2x80x9d, J Amer. Ceram. Soc., 68, C151-153 (1985).), codensification of interlayers and green bodies (C.H. Bates, et al. xe2x80x9cJoining of Non-Oxide Ceramics for High-Temperature Applications,xe2x80x9d Amer. Ceram. Soc. Bull., 69, 350-356 (1990)), hot pressing of suitable silicon carbide powders (T. Iseki, K. Arakawa and H. Suzuki, xe2x80x9cJoining of Dense Silicon Carbide by Hot Pressing,xe2x80x9d J. Mater. Sci. Letters, 15, 1049-1050 (1980)), bonding with polymeric precursors (S. Yajima, et al., xe2x80x9cJoining of Silicon Carbide to Silicon Carbide Using Polyborosiloxane,xe2x80x9d Amer. Ceram. Soci. Bull., 60, 253 (1981)), brazing (J. A. P. Gehris, xe2x80x9cHigh Temperature Bonding of Silicon Carbide,xe2x80x9d M.S. Thesis, New Mexico Institute of Mining and Technology, Socorro, N. Mex. (1989)), reactive metal bonding (S. Morozumi, ete al., xe2x80x9cBonding Mechanism Between Silicon Carbide and Thin Foiuls of Reactive Metals,xe2x80x9d J. of Mater. Sci. 20, 2976-3982 (1985)), xe2x80x9cpressurized combustion reactionxe2x80x9d, reaction with and without the use of tape (H. B. Rabin, xe2x80x9cJoining of SiC/SiC Composites and Dense SiC Using Combustion Reaction in the Tixe2x80x94Cxe2x80x94Ni System,xe2x80x9d J. Amer. Ceram. Soc., 75, 131-135 (1992)), and microwave joining (I. Ahmed and R. Silberglitt, xe2x80x9cJoining Ceramics Using Microwave Energy,xe2x80x9d Mat. Res. Soc. Symp. Proc., 314, 119-130 (1993)). These techniques have limited utility for semiconductor applications due to one or more drawbacks, such as use of filler material which can contaminate the furnace environment, inability of joints to withstand high service temperatures, and the need for very high temperatures or pressures during joining processing. Furthermore, most of these do not concentrate on male/female joints, where, for example, a rod is inserted into a hole and then bonding is performed. Such male/female joints are particularly desirable for fabricating wafer carriers and other furnace components for the semiconductor industry.
A male/female joint broadly defined is a joint in which an inserted (male) member is received within and bonded to a receiving (female) member. An example of a male/female joint is a joint in which the sidewalls of the male and female members are substantially parallel to each other. Such a male/female joint may be a rod inserted, for example, in a receiving closed-end bore or a sheet having parallel sides inserted in a receiving slot or groove. In bonding such a joint, it is desirable that bonding be effected between the sidewalls to provide good stability to the manufactured article. Unlike a butt joint, it is difficult to provide adequate pressure along the sidewalls of the male and female members to secure the male and female members together.
U.S. Pat. No. 5,683,028 to Goela et al. discloses a chemical means of securing a male/female joint in a silicon carbide boat. The boat in composed of four monolithic silicon carbide rods with a plurality of slits or grooves to retain multiple semiconductor wafers for processing. Each rod has two male joint members that slide into a corresponding female port in an endplate to form a single article. The joint is secured with a silicon sealant that provides sufficient pressure along all points or the male/female joint. Optionally, the joint may be further secured with a coating of CVD silicon carbide. Such a joint can withstand the harsh conditions involved in wafer processing and the silicon sealant does not contaminate the environment of the processing apparatus.
Japanese patent publications 2000164522A and Hei 10-45485 disclose a wafer fixture or boat for semiconductor manufacture composed of components coated with silicon carbide and a method of coating. The wafer fixture is composed of rods having a plurality of grooves or slits for holding multiple wafers for processing. The rods have xe2x80x9cTxe2x80x9d shaped catch parts at each end and are secured to endplates having mounting holes by a work body or nut. The rods and endplates of the fixture are not composed of monolithic silicon carbide. The components of the fixture are composed of a silicon matrix with particles of silicon carbide with a thin carbon layer and further coated with a CVD silicon carbide film as described in Hei 10-45485. Such a composition allegedly raises peel resistance of the thin silicon carbide film thus improving oxidation resistance, chemical resistance and thermal shock resistance of the fixture. However, workers in the art know that such thin film silicon carbide coatings often crack or chip when employed in the harsh wafer processing environments. Thus, a wafer fixture composed entirely of CVD silicon carbide is preferred.
Although there are semiconductor wafer fixtures composed of CVD silicon carbide components and that are secured by means that can withstand the harsh conditions of semiconductor wafer processing apparatus, there is still a need for improved CVD silicon carbide wafer fixtures.
The present invention is directed to an apparatus composed of a plurality of silicon carbide rods secured at opposite ends to endplates by a dovetail joint means. Each rod of the apparatus has a dovetail key component that corresponds to a dovetail lock component on an endplate. When the apparatus is assembled, the dovetail joint means secures the rods to the endplates mechanically such that no sealant is required to hold the components of the apparatus together. Each rod has a plurality of grooves for placing semiconductor wafers in the apparatus. The entire apparatus may be placed in suitable chambers for processing the semiconductor wafers.
Advantageously, the dovetail joint provides a sufficiently secure apparatus such that chemical sealing agents within the joint or coating the joint with a chemical sealant may be avoided. Many such sealing agents and chemical coatings may contaminate the apparatus or furnace used to process semiconductor wafers resulting in defective wafers. Additionally, elimination of such sealing agents enables easy and rapid assembly of the apparatus.
Another advantage of the dovetail joint of the present invention is that the dovetail joint need not employ additional fasteners and mounting means such as bolts, clamps or nuts to secure the components of the dovetail joint. Thus, the apparatus has a minimal number of separate parts to function effectively as a wafer boat. A minimal number of separate parts is highly desirable for apparatus used to hold semiconductor wafers. During wafer processing methods, the wafers as well as the apparatus holding the wafers become coated with chemical materials such as silicon or silicon carbide. Such materials are difficult to remove from the holding apparatus during cleaning. When an apparatus such as a wafer boat is composed of numerous parts, especially small fasteners such as bolts, nuts or clamps, cleaning is more difficult and time consuming. Thus, a holding apparatus with a minimal number of separate parts is highly desirable.
The rods and endplates of the apparatus are composed of monolithic silicon carbide. Thus, each component part is a solid material without thin film coatings. Thus, there is no concern for peeling of layers from the rods and endplates of the apparatus. Because the component parts are composed of monolithic silicon carbide, the apparatus is oxidation resistant, chemical resistant and thermal shock resistant. Accordingly, the apparatus may be employed in chambers, such as a RTA, where harsh chemical compounds are employed as well as high temperatures and rapid temperature changes.
A primary objective of the present invention is to provide a semiconductor wafer holding apparatus that is secured by a dovetail joint.
Another objective of the present invention is to provide a semiconductor wafer holding apparatus that can withstand the harsh conditions of chambers used to process semiconductor wafers.
An additional objective is to provide a semiconductor wafer holding apparatus where sealing agents and chemical coating need not be employed to secure the joints of the apparatus.
A further objective of the present invention is to provide a semiconductor wafer holding apparatus that is oxidation resistant, chemically resistant and thermal shock resistant.
After reading the following detailed description of the invention and the appended claims, additional objectives and advantages of the present invention may be ascertained by a person of skill in the art.